wave called secondary peristalsis. Frequently, repetitive secondary peristalsis is required to clear the esophagus of the bolus. Stimulation of the pharynx by the swallowed bolus also produces reflex relaxation of the LES and the most proximal region of the stomach. Thus when the bolus reaches the LES, it is already relaxed to allow passage of the bolus into the stomach. Similarly the portion of the stomach that receives the bolus is relaxed. In addition, esophageal distention produces further receptive relaxation of the stomach. The proximal part of the stomach relaxes at the same time as the LES; this occurs with each swallow, and its function is to allow the stomach to accommodate large volumes with a minimal rise in intragastric pressure. This process is called receptive relaxation (
Fig. 28.8
The LES also has important protective functions. It is involved in preventing acid reflux from the stomach back into the esophagus. An insufficient tonic contraction of the LES is associated with reflux disease, a gradual erosion of the esophageal mucosa, which is not as well protected as the gastric and duodenal mucosa. There is also some evidence that peristalsis in the absence of swallowing (secondary peristalsis) is important for clearing refluxed gastric contents.
1.
The cephalic and oral phases of the meal share many characteristics and prepare the remainder of the GI tract for the meal; these responses are neurally mediated, predominantly by the efferent vagus nerve.
2.
Salivary secretion has important functions and, together with chewing of the food, allows the formation of a bolus that can be swallowed and passed along the esophagus to the stomach.
3.
The ionic composition of salivary secretion varies with the flow rate, which is stimulated during a meal. The primary secretion comes from cells in the acini and is modified by epithelial cells as it passes through the ducts.
4.
Regulation of salivary secretion is exclusively neural; parasympathetic innervation is most important in the response to food.
Catalan MA, et al. Salivary gland secretion. In: Johnson LR, ed. Physiology of the GI Tract. 5th ed. Waltham, MA: Academic Press; 2012.
5.
The swallowing reflex is a rigidly ordered sequence of events that propel food from the mouth to the pharynx and from there to the stomach.
6.
The major function of the esophagus is to propel food from the mouth to the stomach. The esophagus has sphincters at either end that are involved in protective functions important in swallowing and preserving the integrity of the esophageal mucosa.
7.
Esophageal peristalsis (primary) is stimulated by mechanical stimulation of the pharynx, and secondary peristalsis is stimulated by distention of the esophageal wall.
8.
Esophageal function and the associated sphincters are regulated by extrinsic and intrinsic neural pathways.
Lee MG, et al. Molecular mechanism of pancreatic and salivary gland fluid and HCO3 secretion. Physiol Rev. 2012;92:39-74.
Mittal RK. Motor Function of the Pharynx, Esophagus, and its Sphincters. San Rafael, CA: Morgan & Claypool Life Sciences; 2011.
Upon completion of this chapter the student should be able to answer the following questions: 1.
What are the major functions of the monogastric stomach?
2.
What are the gross functional regions of the stomach?
3.
What is the role of the gastric epithelium in digestion and absorption?
4.
What is the role of the proton pump in parietal cell function?
5.
What are some examples of how gastric acid secretion is regulated during the postprandial period?
6.
What are the differences between gastric mucosal protection and defense?